1. Field of the Invention
The present invention relates to a light amplifying optical fiber for an optical fiber amplifier which is used for optical communication.
2. Description of the Prior Art
An optical fiber amplifier is considered a key device for an optical communication system at present and research and development is proceeding. Specifically, research is being performed using as activation ions Pr ions in the 1.3 .mu.m band and Er ions in the 1.5 .mu.m band which are optical signal wavelength bands used for optical communication.
In the constitution of an optical communication system using signal light in the 1.3 .mu.m or 1.5 .mu.m band, optical monitoring systems have been proposed in which 1.65 .mu.m band light is used for monitoring an optical fiber as an optical transmission line. One proposal is described by Y. Koyamada et al. in "Basic Concepts for Fiber Optic Subscriber Loop Operation Systems" (Proceedings of ICC, 341.1, 1990, pp.1541-1544). If the optical monitoring system is used, it is possible to monitor the optical transmission line without stopping the signal light because light having a different wavelength from that of the signal light is used as a probe light. However, there is no semiconductor laser having a high output power which can used as a light source for such a monitoring system. For this reason, a method is proposed in which the output of an existing semiconductor laser is amplified. Also, there is a need for a 1.65 .mu.m band optical fiber amplifier which can be used for the optical monitoring system.
Conventionally, a 1.65 .mu.m band optical fiber amplifier is proposed in which Tm ions are used as the activation ions. For instance, there is "Various Characteristics of Tm-doped Optical Fiber" by Izumi Sankawa, (IEICE Technical Study Report OQE90-85, pp. 89-94). The reason why Tm ions are best as the activation ions in the 1.65 .mu.m band optical fiber is that Tm has energy levels as shown in FIG. 1. That is, when a Tm ion in the .sup.3 H.sub.6 level (ground state) is excited to the .sup.3 F.sub.4 level, the stimulated transition occurs from the .sup.3 F.sub.4 level to the .sup.3 H.sub.6 level so that light emission having the wavelength of 1.65 .mu.m can be obtained. Pump light having wavelengths of 0.7 .mu.m, 0.8 .mu.m, 1.2 .mu.m, and 1.55 .mu.m to 1.6 .mu.m can be used for excitation of Tm ions as shown in the figure. When pump light of 1.55 to 1.6 .mu.m is used, the Tm ions in the .sup.3 H.sub.6 level can be directly excited to the .sup.3 F.sub.4 level. When pump light of 0.8 .mu.m is emitted from a device which can be relatively readily available, the Tm ion in the .sup.3 H.sub.6 level is excited to the .sup.3 H.sub.4 level and relaxed from the .sup.3 H.sub.4 level to the .sup.3 F.sub.4 level. This means that the Tm ion is excited to the .sup.3 F.sub.4 level. As a result, the light emission of 1.65 .mu.m can be obtained through the stimulated transition of .sup.3 F4.fwdarw..sup.3 H.sub.6.
An optical amplifier can be obtained if a typical optical amplifier is constituted using a Tm-doped optical fiber in which the three-level stimulated emission of .sup.3 F.sub.4 .fwdarw..sup.3 H.sub.6 can be utilized, as shown in FIG. 2. The optical fiber amplifier includes a light amplifying optical fiber 1 in which Tm is doped in the core, an optical fiber 2 for signal input, a pump light source 3, a wavelength multiplexer 4 for combining a signal light 2A and pump light 3A supplied from the pump light source 3 for supply to the amplifying optical fiber 1, and a signal light outputting optical fiber 6 for outputting the signal light amplified by the optical fiber 1. Note that in the optical fiber amplifier, an optical isolator 5 may be provided between the light amplifying optical fiber 1 and the signal light outputting optical fiber 6 as necessary.
Absorption spectrum and spontaneous emission spectrum (fluorescent spectrum) as a result of the stimulated emission transition (.sup.3 F.sub.4 .fwdarw..sup.3 H.sub.6) when Tm ions are used as the activation ions in the above optical fiber amplifier are shown in FIG. 3. In the figure, the reference numeral 8 indicate a curve representative of an absorption cross section, and the reference numeral 9 indicates a curve representative of emission cross section. In a case of using this transition, a wavelength at the center of the spontaneous emission spectrum is in the vicinity of 1.8 .mu.m as shown in FIG. 3. Also, a wavelength at the center of the absorption spectrum is 1.65 .mu.m. As seen from these values, much of the pumping energy is used for amplified spontaneous emission (ASE) in the 1.8 to 2.0 .mu.m band. For this reason, the amount of pumping energy is reduced which contributes to amplification of light having the wavelength of 1.65 .mu.m as a target wavelength. Further, a laser oscillation is readily caused because the gain of single traveling path is high in the 1.8 to 2.0 .mu.m band. Once the laser oscillation is caused, the gain cannot be increased in the 1.65 .mu.m band even if the pump light is further inputted.
Conventionally, Tm-doped optical amplifiers are known. Also known are an optical amplifier in which co-doping ions are used as a donor or sensitizer (Japanese Patent Application Laid-Open No. 112576/1994 by Ooishi et al.) and an optical amplifier in which co-doping ions are used as an acceptor (by R. M. Percival et al., "THULIUM DOPED TERBIUM SENSITISED CW FLUORIDE FIBER LASER OPERATING ON THE 1.47 .mu.m TRANSITION", Electron. Lett. Vol. 29, No.12, (1993), pp. 5054-1056. See also Sakamoto et al., "THE EFFECT OF CO-DOPED IONS IN THE 1.4 m BAND Tm DOPED ZBLYAN OPTICAL FIBER AMPLIFIER", Shingaku Gihou LQE 94-12).
A summary of the Laid-Open Japanese Patent Application 112576/1994 is as follows. In order to cause optical amplification laser oscillation in the 1.6 to 2.0 .mu.m band (.sup.3 H.sub.4 .fwdarw..sup.3 H.sub.6) by effectively exciting Tm in the optical fiber, it is desirable to excite with 1.2 .mu.m light. However, a high output laser emitting light having a wavelength of 1.2 .mu.m is not readily available. For this reason, as shown in FIG. 4, ions such as Yb, Er and Dy are doped in the optical fiber together with Tm ions and these co-doped ions are excited by a 1.06 .mu.m high output Nd:YAG laser or a high output semiconductor laser for a 0.98 .mu.m band. The excited co-doped ions transfer energy to the .sup.3 H.sub.5 level of Tm with no light emission. As a result, 1.6 to 2.0 .mu.m optical amplification laser oscillation is made possible.
The summary of the latter references by Percival et al. and Sakamoto et al. is as follows. As shown in FIG. 5, the transition of .sup.3 H.sub.4 .fwdarw..sup.3 F.sub.4 is used for 1.4 .mu.m band amplification. A fluorescent life time is long in the .sup.3 F.sub.4 level as a lower level. For this reason, many ions are excited to the .sup.3 F.sub.4 level and as a result of this it is difficult to form an inverted population. Accordingly, by doping ions of Tb, Ho and Eu in the optical fiber together with Tm ions, the energy is moved from the .sup.3 F.sub.4 level to levels of such ions through a non-emission mechanism. As a result, the number of ions trapped in the .sup.3 F.sub.4 level is decreased, so that it is made possible to form the inverted population. The latter structure aims at the amplification of light having the wavelength of not 1.65 .mu.m but 1.4 .mu.m. Both references are characterized in that the density in a specific energy level of Tm is increased or decreased by use of co-doped ions. On the other hand, in the present invention, the density in a specific energy level is not increased or decreased by doping such ions. Therefore, the present invention is essentially different from the above two references in that the co-doped ions only function to absorb light.
Note that in the references, there are cases in which the energy level .sup.3 F.sub.4 and .sup.3 H.sub.4 are labeled reversely with respect to each other, but the relationship between energy levels is not changed substantially in any case.
Alternatively, a light amplifying optical fiber is proposed in Japanese Patent Application Laid-Open No. 311930/1992) in which ions for absorbing a part the spectrum emitted through the stimulated emission of activation ions in the optical fiber are doped in a region where no activation ions are present or where less activation ions are doped. However, in this proposed light amplifying optical fiber, Nd ions as the activation ions and Yb ions, Sm ions or V ions as the absorption ions are used. Therefore, the present invention is different from the proposal in that the target wavelength is the 1.3 .mu.m band and the wavelength absorbed by Yb, Sm, U is the 1.05 .mu.m band which is not the wavelength being amplified. That is, the proposal is different from the present invention in both object and composition.